how to make light pass through a mesh?
You will want to use the sub surface scattering shader (SSS). or the faster and less accurate translucency shader.

Here is Suzanne with a SSS shader applied, and a very bright light behind her:




The small scale causes the light to spread less far within the object. I set the radius to 1.0,0.6,0.6 to give the light the red cast.